Vitamin K (K comes from "Koagulations-Vitamin" in the German and Scandinavian languages) denotes a group of lipophilic, hydrophobic vitamins that are needed for the posttranslational modification of certain proteins, mostly required for blood coagulation, but also a number of other proteins that chelate calcium ion, and are involved in bone and other tissue metabolism. They are 2-methyl-1,4-naphthoquinone derivatives.

Vitamin K1 is also known as phylloquinone or phytomenadione (also called phytonadione).
Vitamin K2 (menaquinone, menatetrenone) is normally produced by bacteria in the large intestine, and dietary deficiency is extremely rare unless the intestines are heavily damaged, are unable to absorb the molecule, or are subject to decreased production by normal flora, as seen in broad spectrum antibiotic use.

There are three synthetic forms of vitamin K, vitamins K3, K4, and K5, which are used in many areas including the pet food industry (vitamin K3) and to inhibit fungal growth (vitamin K5). Plants synthesize phylloquinone, also known as vitamin K1. Vitamin K1 or phylloquinone is the principal dietary source of vitamin K and its predominant circulating form. Vitamin K2 is the collective term for a group of vitamin K compounds called menaquinones. The menaquinone homolgues are characterized by the number of isoprene residues comprising the side chain.

Vitamin K is found in green leafy vegetables such as broccoli and spinach, and in vegetable oils and cereals. Small amounts can also be found in meat (such as pork), and dairy foods (such as cheese). Vitamin K2 is found in chicken egg yolk, butter, cow liver, certain cheeses and fermented soybean products such as natto. Very little vitamin K is stored by the body; small amounts of this vitamin are deposited in the liver and in the bones, but this amount is only enough to supply the body's needs for a few days. As well as getting vitamin K from food, we also get it from our own bodies because it's produced by bacteria in our intestines.

Vitamin K plays an important role in blood clotting and bone metabolism (carboxylation of osteocalcin). High serum concentrations of undercarboxylated osteocalcin and low serum concentrations of vitamin K are associated with lower bone mineral density and increased risk of hip fracture. Vitamin K supplements may improve bone mass in postmenopausal women. Vitamin K prevents calcification of arteries and other soft tissue. Calcification of organs and other soft tissue is an adverse consequence of aging. Vitamin K may play a role in the regulation of blood sugar. The pancreas, which makes insulin, has the second highest amount of vitamin K in the body.

Vitamin K's primary function is to regulate normal blood clotting (due to its role in the synthesis of prothrombin). Blood clotting is a process that begins automatically when any injury produces a tear in a blood vessel. The process of blood clotting involves a collection of molecules, which circulate continuously through the bloodstream. Vitamin K regulates normal blood clotting by helping the body transport calcium. Protein Z appears to enhance the action of thrombin (the activated form of prothrombin) by promoting its association with phospholipids in cell membranes. Protein C and protein S are anticoagulant proteins that provide control and balance in the coagulation cascade. Vitamin K2, not vitamin K1 (phylloquinone; phytonadione), may improve a group of blood disorders known as myelodysplastic syndromes.

Vitamin K supplements improve bone health and reduce risk of bone fractures, particularly in postmenopausal women who are at risk for osteoporosis. Three vitamin-K dependent proteins have been isolated in bone. Higher vitamin K levels correspond to greater bone density, while low levels of vitamin K have been found in those with osteoporosis. In bones, vitamin K mediates the gamma-carboxylation of glutamyl residues on several bone proteins, notably osteocalcin. Osteocalcin is a protein synthesized by osteoblasts (bone forming cells). The mineral-binding capacity of osteocalcin requires vitamin K-dependent gamma-carboxylation of three glutamic acid residues.

Vitamin K is used to reduce risk of bleeding in liver disease, jaundice, malabsorption, or in association with long-term use of aspirin or antibiotics. Gastrointestinal problems that decrease the absorption of vitamin K, such as obstructions in the bile duct, cystic fibrosis, sprue, Crohn's disease, colitis and medications that reduce the absorption of this vitamin, such as antibiotics. Vitamin K deficiency in those with cystic fibrosis is worsened by their recurrent need for antibiotics. Vitamin K has been used in the treatment of heavy menstrual bleeding, and with vitamin C to treat morning sickness. Newborns are at risk for bleeding in the brain because of the trauma of coming through the birth canal during delivery.

Vitamin K is a fat-soluble vitamin. This means you don't need to have it every day because any of the vitamin your body doesn't need immediately is stored in the liver for future use. You should be able to get all the vitamin K you need by eating a varied and balanced diet. The recommended dietary allowance (RDA) for vitamin K is 80 mg per day for adult males and 65 mg per day for adult females, and 5 mg/day for the newborn infant. Natural forms of vitamin K found in foods are only about half as potent compared to synthetic version. While adequate amount of vitamin K can be obtained by consuming leafy green vegetables on a regular basis for many men and women between 18 and 44 years may benefit from supplemental vitamin K.

Vitamin K deficiency is very rare and occurs when there is an inability to absorb the vitamin from the intestinal tract. Vitamin K deficiency can also occur after prolonged treatment with oral antibiotics. Vitamin K deficiency is often the result of impaired absorption rather than not getting enough in the diet. Prolonged use of antibiotics can also cause a low level of this vitamin because they destroy some of the bacteria in the gut that help to produce vitamin K. Newborns are at risk for vitamin K deficiency. This is because their digestive tracts contain no vitamin K-producing bacteria. Vitamin K deficiency in adults is rare. When it occurs, it is found in people with diseases that prevent the absorption of fat. These diseases include cystic fibrosis, celiac disease, and cholestasis. Symptoms of vitamin K deficiency include easy bruisability, epistaxis, gastrointestinal bleeding, menorrhagia and hematuria.

Although allergic reaction from supplementation is possible, there is no known toxicity associated with high doses of phylloquinone (vitamin K1), menaquinone (vitamin K2), or menadione (vitamin K3) and its derivatives. High intake of vitamin K is not recommended for individuals taking anticoagulant medications such as Warfarin (coumadin).

New research (May 2010) shows that vitamin K protects against non-Hodgkin lymphoma. Non-Hodgkin lymphomas belong to a large group of immune system cancers involving lymphocytes (white blood cells). In 2009, according the National Cancer Institute (NCI) about 65,980 Americans were diagnosed with this form of cancer and almost 20,000 died from the disease. But now scientists at the Mayo Comprehensive Cancer Center in Minnesota think they've found a way to prevent a huge number of these malignancies and the key is vitamin K.

The findings of the study, which were recently announced in Washington, D.C., at the 101st Annual Meeting of the American Association for Cancer Research (AACR), showed that the risk of developing non-Hodgkin lymphoma was slashed dramatically by 45 percent for the study participants who had the highest vitamin K levels compared to participants with the lowest levels of the vitamin. This association remained even after the Mayo research team investigated factors such as age, sex, education, obesity, smoking, alcohol use and consumption of foods with high amounts of antioxidants.